Tánaiste (Irish pronunciation: [ˈt̪ˠaːnˠaʃtʲə]) was the Irish word for the heir of the chief (taoiseach) or king (rí), under the Gaelic system of tanistry. Before independence, the British Lord Lieutenant of Ireland or Viceroy was sometimes referred to in the Irish language as An Tánaiste-Rí, literally 'the deputy king'.

The Taoiseach nominates a member of Dáil Éireann,[6] who will also be a member of the government, to the office. The nominee then receives their seal of office from the President of Ireland in recognition of their appointment. The Tánaiste acts in the place of the Taoiseach during his or her temporary absence. In the event of the Taoiseach's death or permanent incapacitation, the Tánaiste acts in their stead until another Taoiseach is appointed.[7] The Tánaiste is, ex officio, a member of the Council of State. The Tánaiste chairs meetings of the government in the Taoiseach's absence and may take questions on their behalf in the Dáil or Seanad.

Under a coalition government, the Tánaiste is typically the leader of the second-largest government party, just as the Taoiseach is usually leader of the largest; however, during the 1989–92 and the 2007–11 governments, the position was held by a Fianna Fáil member, although they were in coalition.